1. Field of the Invention
This invention relates to a misfire-detecting system for internal combustion engines, and more particularly to a misfire-detecting system of this kind, which is adapted to detect a misfire attributable to the fuel supply system.
2. Prior or Art
An internal combustion engine has spark plugs provided for cylinders for igniting a mixture of fuel and air drawn into the respective cylinders. In general, high voltage (sparking voltage) generated by the ignition coil of the engine is sequentially distributed to the spark plugs of the cylinders of the engine via a distributor, to ignite the air-fuel mixture. If normal ignition does not take place at one or more of the spark plugs, i.e. a misfire occurs, it will result in various inconveniences such as degraded driveability and increased fuel consumption. Furthermore, it can also result in so-called after-burning of unburnt fuel gas in the exhaust system of the engine, causing an increase in the temperature of a catalyst of an exhaust gas-purifying device arranged in the exhaust system. Therefore, it is essential to prevent occurrence of a misfire. Misfires are largely classified into ones attributable to the fuel supply system and ones attributable to the ignition system. Misfires attributable to the fuel supply system are caused by the supply of a lean mixture or a rich mixture to the engine, while misfires attributable to the ignition system are caused by failure to spark (so-called mis-sparking), i.e. normal spark discharge does not take place at the spark plug, due to smoking or wetting of the spark plug with fuel, particularly adhesion of carbon in the fuel to the spark plug, which causes current leakage between the electrodes of the spark plug, or abnormality in the ignition system.
The present assignee has already proposed a misfire-detecting system for detecting misfires attributable to the fuel supply system, which comprises sparking voltage detecting means, and misfire-determining means which determines occurrence of a misfire based on results of comparison between the detected value of the sparking voltage and a predetermined reference value (Japanese Provisional Patent Publication (Kokai) No. 4-279768), and further a misfire-determining system of this kind which comprises sparking voltage-detecting means, and misfire-determining means which determines that a misfire has occurred when a time period over which the detected value of the sparking voltage exceeds a predetermined voltage value or a value proportional to an area of a portion of the detected sparking voltage exceeding the predetermined voltage value exceeds a reference value (U.S. Ser. No. 07/846,238 filed Mar. 5, 1992 based on Japanese Patent Application No. 3-67940).
However, the above proposed system does not specify a manner of setting the predetermined value or the reference value for determining occurrence of a misfire. Therefore, there remains a problem of erroneously determining occurrence of a misfire when the amount of ions generated in the combustion chambers is small even during normal combustion. For example, when the temperature of the combustion chambers is low or the temperature of the air-fuel mixture is low, the amount (density) of ions generated by combustion is small. In such cases, it can not be determined that a misfire has occurred, unless the reference value for determining occurrence of a misfire is set depending on operating conditions of the engine.
Further, when the engine is in particular operating conditions, e.g. when fuel supply to the engine is resumed after fuel cut, or when the engine is being started or has just been started, there is a high possibility that a misfire is erroneously determined to have occurred.
However, the proposed system does not take these problems into consideration.
In the proposed system, misfire detection is carried out even when the engine is in a transient state from air-fuel ratio feedback control based upon an output from an oxygen concentration sensor arranged in the exhaust system of the engine to air-fuel ratio open-loop control, or vice versa (i.e. upon termination of the air-fuel ratio feedback control or upon starting of same). However, in such a transient state, combustion of the mixture becomes unstable, making it difficult to set a suitable reference value for determining a misfire, which results in a high possibility that normal combustion is erroneously determined to be a misfire. However, the above proposed system is intended not to detect a misfire occurring under a temporary or transient engine operating condition in which combustion becomes inevitably unstable, but to detect a constantly-occurring misfire caused by faulty operation of the engine, particularly the fuel supply system. Therefore, even if the system detects a temporary misfire in such a transient state of the engine as described above, it is unable to affirmatively determine that a misfire is occurring.
Further, a similar problem arises when an engine provided with an exhaust gas recirculation system is in a transient state from a state in which the exhaust gas recirculation is being carried out to a state in which it is inhibited or vice versa (i.e. upon termination of the exhaust gas recirculation control or upon starting of same), since the air-fuel ratio of the mixture and the ignition timing undergo temporary changes in this transient state of the engine as well.
If the engine is provided with a valve timing changeover device for changing valving characteristics (hereinafter referred to as "valve timing") of intake valves and/or exhaust valves (timing of opening and closing of the valves, and/or valve lift amount), a similar problem arises when the valve timing is changed, since the amount of fuel supply and a basic ignition timing advance value (which is set according to the engine rotational speed and load on the engine) are changed in response to changing of the valve timing, and hence combustion may become temporarily unstable.
Further, the proposed misfire-detecting system suffers from an inconvenience that if the air-fuel ratio of a mixture supplied to the engine has changed, the reference value for determining occurrence of a misfire can become unsuitable for the determination, to make it impossible to accurately determine occurrence of a misfire.
More specifically, the air-fuel ratio of the mixture is controlled to values suitable for operating conditions of the engine. In other words, generally, the fuel supply control is not carried out with the air-fuel ratio held at a constant or fixed value, but in a normal air-fuel ratio feedback control region, the air-fuel ratio is feedback-controlled to a stoichiometric value (e.g. 14.7) in response to the output from the oxygen concentration sensor, whereas the air-fuel ratio is corrected to a richer value with respect to the stoichiometric value when the temperature of the engine (e.g. the temperature of engine coolant) is low, and to a leaner value with respect to same for the purpose of reducing fuel consumption when the engine is operating in a low load condition. If the air-fuel ratio is varied in this manner, the density of ions generated by combustion of the mixture which determines the reference value also changes, so that the reference value may deviate from a proper value for determining occurrence of a misfire if it is not set with this variation in the air-fuel ratio taken into consideration, and hence an accurate misfire detection becomes impossible to carry out.
Further, in the above proposed system, misfire determination is carried out irrespective of whether there is abnormality in sensors for detecting engine operating parameters, such as the engine rotational speed and the engine load, as well as in wiring connecting the sensors to a control unit. Consequently, there is a possibility that a misfire is erroneously determined to have occurred.
Further, aging or failure of fuel injection valves and a fuel pressure regulator of the engine (the fuel supply system), and the oxygen concentration sensor arranged in the exhaust system can result in inaccurate air-fuel ratio feedback control based upon the output from the oxygen concentration sensor, causing a deviation of the air-fuel ratio from a desired value. In such an event, it is impossible to accurately detect the actual air-fuel ratio determined by the combustion state, and hence to set a proper reference value for determining occurrence of a misfire, which makes it difficult to accurately determine occurrence of a misfire.